Oh yeah, I did play Soul Calibre 2 but I recall it being more like side-stepping.
I guess what I was thinking of is something that'd pretty much necessitate a split-screen view, with you having absolute control over your guy in the arena, maybe similar-looking to Assassin's Creed. That or some very clever camera work that only splits off when absolutely necessary (ie: very far apart).
I realise this stretches the definition of what a beat 'em up actually is, but it seems like it'd be a really interesting under-explored direction.